The present invention relates to a process for improving the properties of polymers, and more specifically, of thermoplastic polymers. The well known Diels-Alder or diene reaction is utilized to extend or increase the molecular weights of the polymers and/or to crosslink or cure the thermoplastics.
Thermoplastic polymers have certain well known advantages and disadvantages relative to the thermosetting variety. Several advantages of thermoplastic polymers include better resilience or toughness, longer fatigue life and greater impact resistance. Several disadvantages of thermoplastic polymers relative to the thermosetting variety are that thermoplastic polymers are not normally cured nor crosslinked like the thermosets. In addition, the thermoplastics are usually sensitive to certain common solvents which can swell or even dissolve plastics including acetone, alcohols, hydrocarbons, methylene chloride, chloroform, and other halogenated hydrocarbons, fuels such as JP-4 jet fuel, and hydraulic fluids such as tricresyl phosphate. Furthermore, because thermoplastics are linear, they tend to creep or elongate under load at elevated temperatures greater than 50.degree. C.
Active unsaturated compounds, such as maleimides, have been reported to bring about the crosslinking of certain unsaturated polymers, including natural rubber [P. Kovacic and R. W. Hein, J. Amer. Chem. Soc., 81, 1190 (1959)]. However, that type of free radical-induced vulcanization is, by nature, very random and slow, requiring a peroxide catalyst for the crosslinking to occur at reasonably low temperatures. And, in contrast to the invention, the peroxide-catalyzed reaction, due to its non-selective quality, cannot be used to extend or increase the molecular weights of polymers in a linear manner.